These studies seek to better assess the effects of environmental agents on spermatogenesis and male reproductive capacity. Mechanisms of toxicity are studied and new approaches to toxicity testing are proposed and validated. In addition, selected environmental agents are tested for reproductive toxicity. The following studies are ongoing: (1) defining the role of germ cell DNA repair as an indicator and modifier of toxicity; (2) modeling the various factors which affect testicular toxicity; and (3) investigating the toxication-detoxication of polycyclic hydrocarbons in gonadal tissues using in vitro and perfused testis preparations. To date, selected mutagens, trace metals, TCDD, nitrous oxide, Freon 22, and dibromochloropropane (DBCP) have been studied. Alkaline elution analysis of germ cell DNA appears to be a reliable indicator of damage and repair as well as a predictor of toxicity. The inter-relationship of the blood-testis barrier, toxication-detoxication reactions, and DNA repair have been described. These various studies contribute to an increased undestanding of mechanisms of toxicity and better laboratory tests as well as more reliable extrapolation of laboratory data to man and improved estimates of human risk.